Various systems employ sensing devices which are extremely sensitive to infrared radiation. However, in many situations the system and infrared sensors must be maintained in a protected environment and the infrared energy admitted to the protected environment through an external window. Typical of such situations is an airborne system wherein the sensors are located in an airplane and infrared energy is sensed through an external window.
Not only must the window possess sufficient physical strength to withstand rapidly changing pressure and temperature differentials, the external surface of the window must withstand abrasion by rain, air and dust. Furthermore, in order to permit all-weather operation, means must be provided to prevent the formation of ice on the window and, for military applications, the window must provide a shield against longer wavelength electromagnetic radiation such as radar and radio. ,
Compositions of zinc selenide/zinc sulfide are currently used as infrared windows because they are substantially transparent to infrared wavelengths and can be formed into bodies having sufficient physical strength to withstand the pressure differentials required for airborne windows. Other materials such as germanium and gallium arsenide have been proposed for use as external windows since these materials are substantially more transparent in the long infrared (6-5 .mu.m) than the zinc selenide/zinc sulfide compositions. However, all these materials are prone to erosion and abrasion from the atmospheric conditions encountered and are transparent to most radar and radio wavelengths. Thus, unless special shielding is provided, they do not shield the aircraft interior from longer wavelength penetration. Furthermore, most prior window materials cannot be provided with de-icing capabilities without seriously affecting their performance as windows for infrared radiation.